1. Field of the Invention
This invention relates to improvements in optical add/drop wavelength division multiplex (WDM) systems and more particularly to the control of add channel level in such systems. the invention has particular application in underwater cable systems employing optical fibre cables.
In WDM transmission systems several wavelengths are transmitted along a main trunk which is provided with one or more branching units each of which is arranged to route one or more particular wavelength channels to an individual branch terminal, so called drop channels. The branch terminal is arranged to add one or more channels of the same or different wavelengths to the trunk. Such systems require the control of power levels of the added channels under normal operating and fault conditions to ensure that individual channels are not excessively impaired compared to other channels. In the case of branches which include saturated optical repeater amplifiers which introduce several dBs of compression the delicate balance of optical powers in individual channels can easily be upset or irrevocably impaired. The present invention seeks to provide simple adjustment or control of the amplitude of add channels provided by such an amplifier.